A truss Spar with direct vertical access merit has been designed and installed in ultra-deep water of Gulf of Mexico for a few years. This paper intends to carry out a comparative study of a Paired-Column Semi-submersible (PC-Semi) platform with the same topsides payloads, TTRs tensions, Ram-style tensioners with 28-ft stroke range and same SCRs tensions in the same metocean criteria with same global performance and mooring design criteria.

PC-Semi hull form development and evolution for ultra-deep water dry tree applications in central Gulf of Mexico have been undertaken for a few years through RPSEA ultra-deep water program. Extensive and comprehensive model testing including wind tunnel tests, wave basin tests and VIM tests has been completed successfully. Feasibility and advantages of PC-Semi for dry tree application with large topsides payloads in central GoM have been published and established for years. PC-Semi platform Approval in Principal has been awarded by DNV.

Application of PC-Semi concept as an alternative to Spar for West GoM with fewer top tension riser counts and less topsides payloads than those in central GoM was carried out in this study to identify advantages of hull form and scalability of PC-Semi concept. PC-Semi hull and mooring configurations are sized and validated through comprehensive global performance analysis consisting of offsets, rotations, vertical motions, minimum airgaps, maximum accelerations and TTR strokes and mooring analysis. After validation analysis, key figures of hull and mooring configurations along with the detailed global performance results of PC-Semi are summarized and discussed. Features of pre-service conditions, such as feasibility of vertical wet tow and quayside integration, of both hull forms are also addressed and highlighted.

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